How Much Chopped Onion from 1 Onion Conversion
1 medium onion = 1 cup chopped = 150g. 1 large = 1.5 cups.
One medium onion yields about 1 cup of chopped onion, or 150 grams. A large onion gives you 1.5 cups (225g), while a small onion produces 0.5 cup (75g). These measurements assume a standard 1/4-inch dice.
Onion sizes vary wildly at the grocery store, which makes recipe planning tricky. When a recipe calls for "1 onion, chopped," most cookbook authors mean a medium onion unless they specify otherwise. The problem: your medium might be someone else's large. Weight solves this. A medium onion weighs 8 ounces (225g) whole, before peeling and trimming.
Professional kitchens measure onions by weight after chopping because it's the only accurate method. The same onion chopped fine versus chopped coarse can differ by 20% in cup measurements due to how the pieces pack together.
How to Convert
Start with the onion's whole weight to predict your yield. A 225g (8 oz) medium onion loses about 10% to peeling and trimming, leaving 200g. After chopping and accounting for moisture loss, you get 150g or 1 cup.
For different dice sizes: fine dice (1/8 inch) packs tighter and yields about 0.9 cups from a medium onion. Coarse chop (1/2 inch) yields 1.1 cups because of air gaps. Minced onion compresses to 0.75 cups.
Quick reference: Small onion (4-5 oz whole) = 0.5 cup chopped. Medium onion (8 oz whole) = 1 cup chopped. Large onion (12 oz whole) = 1.5 cups chopped. Jumbo onion (16+ oz whole) = 2 cups chopped.
Common Mistakes
Using cup measurements for onions in recipes where precision matters. French onion soup needs 2 pounds of onions, not "4 large onions." Your large might be half the size of the recipe developer's. Measuring chopped onion too soon after cutting. Fresh-chopped onion releases moisture for 5-10 minutes, changing both weight and volume. Let it sit before measuring for accuracy. Not accounting for cooking reduction. Raw chopped onion shrinks by 75% when caramelized. One cup raw becomes 1/4 cup caramelized.
Pro Tips
Buy onions by weight when recipe accuracy matters. Three pounds of onions is always three pounds, regardless of how many individual onions that means. For meal prep, chop onions in bulk and freeze in 1-cup portions (150g each) in freezer bags. Press flat for quick thawing. Use within 3 months.
Yellow onions are the default for most recipes. They average 8 ounces each. Red onions run slightly smaller at 6-7 ounces. White onions are often larger at 10-12 ounces. Sweet onions like Vidalia can be huge, up to 1 pound each.
Ingredient-Specific Notes
Yellow onions
The workhorse onion. Medium specimens weigh 8 ounces whole and yield 1 cup chopped. Sharp when raw, sweet when cooked. Standard choice for stocks, soups, and sautés. Storage onions last 2-3 months in a cool, dry place.
Red onions
Typically smaller than yellow, averaging 6-7 ounces. One medium red onion yields 0.75 cup chopped. Milder raw flavor makes them ideal for salads and sandwiches. Color bleeds when cooked, turning dishes pinkish.
White onions
Crisper texture than yellow onions. Medium size is 10 ounces, yielding 1.25 cups chopped. Higher water content means they cook faster and caramelize less effectively. Mexican cuisine standard.
Shallots
Not technically onions but used similarly. One large shallot (2 ounces) equals 1/4 cup chopped. Substitute 1/4 cup minced shallot for 1/3 cup minced onion in recipes. Sweeter and more complex than onions.
Green onions (scallions)
One bunch (6-8 stalks) yields 1 cup chopped, using white and light green parts. Dark green tops add color but less flavor. 3 green onions = 1/4 cup regular onion in cooked dishes.
Frequently Asked Questions
How many onions do I need for 2 cups chopped?
Two medium onions or 1.5 large onions. For accuracy, buy 1 pound of onions, which yields 2.5 cups chopped after peeling and trimming. If the recipe is forgiving (like soup or stew), round to 2 medium onions. For dishes where onion ratio matters (like French onion soup or onion jam), weigh them. The 0.5 cup difference between two small onions and two large ones can change the dish's flavor balance.
Can I substitute dried minced onion for fresh?
Yes, at a 1:3 ratio. One tablespoon dried minced onion equals 3 tablespoons (1/4 cup) fresh chopped onion. Rehydrate in warm water for 15 minutes before using in dishes that won't cook long. For sautéing, add dried onion directly to oil. It absorbs fat and rehydrates while cooking. Dried onion works poorly in raw applications like salsa or salad.
How long do chopped onions last?
Refrigerated in an airtight container: 7-10 days. Frozen: 3 months with minimal flavor loss, 6 months maximum. Fresh-cut onions release sulfur compounds that intensify over time, making stored onions taste sharper than fresh. For best quality, chop just before cooking. If prepping ahead, submerge chopped onions in cold water to slow enzymatic reactions. Drain and pat dry before using.
Why do recipes call for onions by count instead of weight?
Home cooking tradition. Most recipes predate widespread kitchen scale use. Onions were standardized sizes at markets 50 years ago, making "1 medium onion" reliable. Modern onions vary more due to year-round growing and multiple varieties. Professional recipes specify "8 ounces onion, diced" or "2 cups diced onion (300g)." Cookbook publishers keep the count system because home cooks expect it.
What's the difference between diced, chopped, and minced onion?
Size and uniformity. Diced: precise 1/4-inch cubes, yields 1 cup per medium onion. Chopped: rough 1/4 to 1/2-inch pieces, yields 1.1 cups due to irregular packing. Minced: tiny pieces under 1/8 inch, yields 0.75 cup because it compresses. Fine dice (1/8 inch) falls between diced and minced. Recipe texture depends on cut size. Minced onion disappears into sauces. Diced holds shape in soups.